This invention relates generally to serial matrix printers and more particularly to a circuit for generating timing pulses to control the firing of print elements in such printers while moving in either direction along a print line.
In serial matrix printers, characters are formed along a print line from a matrix of selectively disposed dots produced on the recording medium by selectively energized print elements. The print elements can be impact wires or electrodes which cause a change in the color of the recording surface thermally or by removal of portions of a surface layer. To achieve good printing quality, the times at which the print elements are energized is critical to align successive rows of dots. In addition, the elements are frequently operated in both the forward and reverse direction, and hence timing must be made the same in either direction of travel.
Both the indication of direction of travel of the print head and timing pulse generation for the print elements are obtained through the application of a pair of signals produced in phase quadrature to an appropriate logic circuit. Examples of generation of direction signals and timing signals are found in U.S. Pat. Nos. 4,225,251 and 4,195,938. In the first patent, the respective successions of output signals from the two transducers are logically combined to determine direction of motion of the print head, and print element control signals are generated at each edge of each pulse in each succession of pulses. In the second patent, transducers likewise are placed relative to a band of alternating transparent and opaque windows to produce successions of pulses in phase quadrature for purposes of determining direction, but the print element timing signals are generated from only leading edge of only one of the successions of pulses.